1. Technical Field of the Invention
This invention relates to an image formation system such as a printer, a facsimile, or a copier for forming an image using an electrophotographic technology and an image support apparatus used with the image formation system.
2. Related Art
Generally, an image formation system using electrophotographic technology comprises a image supporter having a photosensitive layer on an outer peripheral surface, charge means for uniformly charging the outer peripheral surface of the image supporter, exposure mechanisms for selectively exposing the outer peripheral surface uniformly charged by the charge means for forming an electrostatic latent image, developing means for giving toner of a developer to the electrostatic latent image formed by the exposure mechanisms for rendering the image visible as a toner image, and transfer means for transferring the toner image developed by the developing means to a transfer medium such as paper.
A hard photosensitive drum formed with a photosensitive layer on an outer peripheral surface and a flexible photosensitive drum formed with a photosensitive layer on a surface are generally known as the photosensitive bodies.
Roller-like means brought into contact with the surface of the image supporter are known as the charge means, the developing means, and the transfer means. Hard rollers and soft rubber rollers are known as the rollers.
In using a hard photosensitive drum as the image supporter and a hard roller as the roller brought into contact with the hard photosensitive drum, there are limits in manufacturing the hard photosensitive drum and the hard roller with high accuracy, such that an error always occurs, and it is difficult to bring both the drum and roller into uniform contact with each other. If they do not come in uniform contact with each other, a local gap occurs, causing charge, developing, or transfer unevenness, or the photosensitive drum and the hard roller are pressed against each other more strongly than necessary so as to be damaged.
Therefore, usually if the image supporter or the roller brought into contact with it is made of a hard substance, the other is not made of a hard substance. That is, for a hard photosensitive drum as the image supporter, the roller is made of soft rubber; and for a hard roller as the roller, a flexible photosensitive belt is used as the photosensitive body.
However, if the roller brought into contact with the image supporter is made of soft rubber, there are other problems.
Specifically, to make a charge roller, which is to be brought into contact with an image supporter, out of a rubber roller, conductive particles of carbon, etc., are dispersed to make the roller conductive. However, the rubber hardness changes due to unevenness or variations in the carbon dispersion degree, and the hardness on the roller surface varies, so that an intimate contact state with the image supporter is not produced.
In contrast, if the carbon dispersion amount is lessened to produce the intimate contact state with the image supporter, conductivity varies, causing charge unevenness.
If a roller to which a plasticizing agent is added as a compounding agent is used to enhance flexibility, the plasticizing agent may seep out to the surface because of long-term use or the use environment. The plasticizing agent is deposited on a photosensitive body, changing the characteristic of a photoconductive material in the photosensitive body or causing the photosensitive body stick to the roller and the surface of the photosensitive body to peel off.
Such problems can be solved by using a hard roller as the roller and a flexible photosensitive belt as the photosensitive body.
However, to use a photosensitive belt as the photosensitive body, at least two support rollers are required to support the photosensitive belt, thus the structure becomes complicated and in addition the system is larger in size.
Previously, a photosensitive drum described in Japanese Patent Publication No. Hei 4-69383 has been known as means for solving all the problems described above.
FIGS. 48-50 show the photosensitive drum described in Japanese Patent Publication No. Hei 4-69383.
The photosensitive drum 1 comprises a rotation shaft 2, an elastically-deformable, elastic material layer 3 supported on the rotation shaft 2 and taking the shape of a cylinder in a free state, and an outer layer 4 attached surrounding the elastic material layer 3. The outer layer 4 comprises a photosensitive body support layer 5 that can be elastically deformed and a photosensitive layer 6 supported on a surface of the support layer 5. The elastic material layer 3 fills the space between the rotation shaft 2 and the outer layer 4 without forming any substantial gap.
Since the photosensitive drum 1 has the outer layer 4 and the elastic material layer 3 that can be elastically deformed, when an external force is exerted on the surface of the photosensitive drum 1, the surface can be elastically deformed.
In FIG. 48, numeral 7 is a charger, numeral 10 is a developing roller, and numeral 13 is a transfer charger.
At the image formation time, the photosensitive drum 1 is rotated clockwise in FIG. 48 and the photosensitive layer 6 of the drum 1 is charged to a predetermined polarity by the charger 7. The charge area is irradiated with light 8 for forming an electrostatic latent image on the drum 1. The latent image is developed with toner supported on the developing roller 10 rotating in the arrow direction in FIG. 48 as a visible image and is transferred to transfer paper 12 by the transfer charger 13.
In FIG. 48, numeral 14 is a separation charger, numeral 15 is a cleaning blade, and numeral 16 is a static elimination charger.
According to this configuration, since the surface of the photosensitive drum 1 can be elastically deformed, the developing roller 10 can be pressed against the photosensitive drum 1 for elastically deforming the surface of the photosensitive drum 1 in the radial direction thereof. Thus, if the peripheral surfaces of the photosensitive drum 1 and the developing roller 10 are a little eccentric relative to the center axis, the outer diameters of the photosensitive drum 1 and the developing roller 10 vary a little on manufacturing, or at least the surface of the developing roller 10 is made of a rigid body, toner on the developing roller 10 can be brought into contact with the photosensitive drum 1 in a more reliable and stable state than was previously possible without involving trouble of damaging the drum surface, the developing roller, etc., and degradation of image quality of a visible image caused by a large gap produced between the toner on the developing roller 10 and the surface of the photosensitive drum 1 can be suppressed.
That is, according to the photosensitive drum 1, even if a hard developing roller is used, damage to the photosensitive drum or the developing roller can be prevented and the system can also be prevented from being too large.
A photosensitive drum like the photosensitive drum described above is also disclosed in Unexamined Japanese Patent Publication No. Sho 58-90655.
On the other hand, Unexamined Japanese Patent Publication No. Sho 58-86550 discloses a drum-like photosensitive member comprising an endless belt made of a nonmagnetic metal 0.01-2 mm thick prepared by an electric casting method as a drum base body 31 (see FIG. 51), an image support layer (photoconductive material layer) 32 formed on the drum base body 31, and disk-like end plates 33 for supporting the drum base body 31 at both ends thereof, for saving weight and preventing an inductive eddy current from occurring.
The photosensitive drum 1 described in Japanese Patent Publication No. Hei 4-69383 (see FIGS. 48-50) has the following problem due to the fact that the elastic material layer 3 fills the space between the rotation shaft 2 and the outer layer 4 without forming any substantial gap.
The photosensitive layer 6, which is formed on the elastic material layer 3, is axially displaced by a minute force. Press members such as the developing roller 10 and the cleaning blade 15 pressed against the photosensitive layer 6 are disposed on the photosensitive layer 6. Thus, if the rotation axis of the photosensitive layer 6, the axes of the press members, etc., are inclined or the press force is uneven in the axial direction, the photosensitive layer 6 is subjected to an axial thrust force and is axially displaced by the thrust force. Since the thrust force fluctuates, an image formed on the photosensitive layer 6 is also axially displaced. Axial image position accuracy is therefore degraded. Particularly, to superimpose multiple colors, degradation of color superimposing accuracy results in a hue shift, causing an image to be degraded significantly.
The photosensitive drum 1 can be manufactured by alternative methods of:
(1) first preparing the outer layer 4 formed with the photosensitive layer 6 on the photosensitive body support layer 5, next placing the rotation shaft 2 and the outer layer 4 with a predetermined spacing therebetween and pouring a heated elastic material into the space between the rotation shaft 2 and the outer layer 4 for forming the elastic material layer 3;
(2) first placing the rotation shaft 2 and the photosensitive body support layer 5 with a predetermined spacing therebetween and pouring a heated elastic material into the space between the rotation shaft 2 and the photosensitive body support layer 5 for forming the elastic material layer 3, next forming the photosensitive layer 6 on the photosensitive body support layer 5; or
(3) preparing a tubular elastic member having an outer diameter a little larger than the inner diameter of the outer layer 4 and inserting the tubular elastic member into the outer layer 4 in diametrically compressed relation, thereby forming the elastic material layer 3.
However, in method (1), a heated elastic material is poured into the outer layer 4 with the photosensitive layer 6 formed on the surface of the outer layer 4, thus the photosensitive body characteristics are degraded due to heat, etc. There is a fear of damage to the surface of the photosensitive layer 6 and deposition of a foreign material of elastic material, etc., thereon.
In method (2), the elastic material layer 3 is formed before the photosensitive layer 6 is formed. Thus, the elastic material layer 3 swells, melts, or hardens because of a cleaning fluid and a coating liquid at the photosensitive layer coating time. As a result, it is feared that the elastic material layer may not function properly.
Therefore, it is extremely difficult to provide a desired photosensitive drum 1 by method (1) or (2).
In method (3), when the tubular elastic member is released from the compression state and swells to the outer layer 4, it is feared that the tubular elastic member swells nonuniformly. Thus, it is feared that the concentric degree of the rotation shaft 2 and the outer layer 4 with each other may be impaired and that when the photosensitive body 1 rotates, it may shake or swing very largely. In the image formation system, the photosensitive body is surrounded by the abutment members such as the charge means, the developing means, the transfer means, and the cleaning means abutting the photosensitive body. Thus, if the photosensitive body shakes or swings largely, the contact state between the photosensitive body and the abutment members becomes unstable and image unevenness occurs.
In recent years, the main components of the image formation system have been put into units, but this is not mentioned at all in Japanese Patent Publication No. Hei 4-69383.
On the other hand, in the drum-like image support member disclosed in Unexamined Japanese Patent Publication No. Sho 58-86550 (see FIG. 51), if the drum base body 31 can be easily bent inwardly, the drum base body 31 can be used as artificial soft material; it can be expected that the problems in the photosensitive drum 1 described in Japanese Patent Publication No. Hei 4-69383 (see FIGS. 48-50) are solved.
However, use of the drum base body 31 as artificial soft material is not mentioned at all in Unexamined Japanese Patent Publication No. Sho 58-86550.
Moreover, the drum-like image support member (see FIG. 51) has a structure directly supporting the drum base body 31 at both ends thereof simply on the disk-like end plates 33. Thus, if an attempt is made to use the drum base body 31 as artificial soft material, the following problem arises:
Fit tolerance exists between the drum base body 31 and the disk-like end plates 33. If an attempt is made to fix the drum base body 31 to the end plates 33 by bonding, etc., the drum base body 31 floats partially from the peripheral surfaces of the end plates 33 by the tolerance. Thus, if the peripheral surfaces of the end plates 33 are high in roundness, the roundness of the drum base body 31 is degraded and the image support layer (photoconductive material layer) 32 shakes or swings largely, making it extremely difficult to provide a reliable and stable contact state with an abutment member such as a hard roller. If the fit tolerance is made extremely small, the problem is corrected somewhat. However, if the fit tolerance is made extremely small, it becomes extremely difficult to fit the drum base body 31 to the end plates 33, namely, to manufacture the unit.
A photosensitive body drive as described in Unexamined Japanese Patent Publication No. Hei 4-188164 in known as a means for meeting such demands.
FIG. 52 shows the photosensitive body drive; FIG. 52 (a) is a side view and FIG. 52 (b) is a perspective view.
The photosensitive body drive comprises a photosensitive belt 20 formed as a tubular thin sheet, a drive roller 21 having an outer diameter peripheral length shorter than the inner diameter peripheral length of the photosensitive drum 20 and being placed inside the photosensitive belt 20 for rotating, and press members 22 having a coefficient of friction with the photosensitive belt 20 set smaller than the coefficient of friction between the drive roller 21 and the photosensitive belt 20 for pressing the photosensitive belt 20 slidably while bringing the photosensitive belt 20 into intimate contact with the drive roller 21 in a predetermined range in the circumferential direction of the drive roller 21. In FIG. 52, numeral 24 is a charger, numeral 25 is an exposure device, numeral 26 is a developing roller, numeral 27 is a transfer charger, and numeral 28 is a cleaning roller.
According to the photosensitive body drive, the photosensitive belt 20 is driven in a state in which it is brought partially into intimate contact with the surface of the drive roller 21 by the press members 22, and warp 23 is formed in the portion where the press members 22 do not exist because of the peripheral length difference from the drive roller 21.
Thus, the photosensitive belt 20 can be used as a hard material in the intimate contact portion produced by the press members 22 because the hardness of the photosensitive belt 20 is simulated by the hardness of the drive roller 21, and can be used as an elastic body in the portion where the press members 22 do not exist because the warp 23 is formed.
Therefore, according to the drive, the cleaning roller 28 made of an elastic body can be brought into contact with the intimate contact portion of the photosensitive belt 20 with the press member 22 and the developing roller 26 made of a hard material can be brought into contact with the warp 23 portion.
If the developing roller 26 is made of a hard material, it is brought into contact with the warp 23 portion stably with a sufficient nip width and by a very low press contact force because the warp 23 acts as an elastic body.
That is, according to the drive, even if a hard developing roller is used, damage to the photosensitive body or the developing roller is prevented and the drive can also be prevented from being too large.
Drives similar to the drive described above are also disclosed in Unexamined Japanese Patent Publication Nos. Hei 6-27859 and 6-258989, for example.
In the photosensitive body drive described in Unexamined Japanese Patent Publication No. Hei 4-188164, the photosensitive belt 20 is guided by the press members 22 disposed partially in the margins of the photosensitive belt 20. Thus, when the photosensitive belt 20 enters the press member 22, a bend stress easily occurs in the margin (end margin) of the photosensitive belt 20 at an entrance portion 22a, whereby a break, a crack, or peeling-off of a photosensitive layer easily occurs in the end margin of the photosensitive belt 20 (inferior durability). Thus, it is feared that the photosensitive belt 20 may be destroyed from the end margin or that a fatal image defect may be caused even if the photosensitive belt 20 is not destroyed.
The photosensitive belt 20 must be handled as a single unit until it is built in the drive in a state as shown in FIG. 52. However, the photosensitive belt 20 is formed like a tubular thin sheet as described above and does not have sufficient rigidity, thus it is difficult to handle.
Further, in the drive, the warp 23 is formed in the portion where the press members 22 for pressing both ends of the photosensitive belt 20 do not exist. Thus, openings 29 are made between the photosensitive belt 20 and the drive roller 21 at both ends of the warp 23 formation portion.
Thus, foreign materials such as a toner, a toner external additive, paper powder, etc., floating in the drive easily enter the space between the photosensitive belt 20 and the drive roller 21 through the openings 29, 29. As the foreign materials enter, the friction force between the photosensitive belt 20 and the drive roller 21 lowers and it is feared that the photosensitive belt 20 will not be driven.
On the other hand, in the photosensitive body drive described above, the developing roller 26 is brought into contact with the in the warp 23 portion where the press members 22 do not exist. Thus, the drive roller 21 or any other vibration source causes the photosensitive belt 20 to vibrate, particularly in the radial direction of the photosensitive belt 20 in the abutment portion against the developing roller 26. When the photosensitive belt 20 vibrates, the abutment state against the developing roller 26 becomes unstable, causing jitter or inconsistencies in density in a formed image.
Since the conventional drive drives the photosensitive belt 20 by the friction force between the photosensitive belt 20 and the drive roller 21 placed in the photosensitive belt 20, it is feared that the photosensitive belt 20 will not necessarily be driven reliably.